Near-Perfect Particle Measurement Achieved

Below:

Next story in Science

The mind-bending laws of quantum mechanics say we can't observe
the smallest particles without affecting them. Physicists have
now caused the smallest-ever disturbance while making a quantum
measurement — in fact, almost the minimum thought to be possible.

This disturbance is called back-action, and it is one of the
hallmarks of
quantum mechanics, which governs the actions of the very
small. It arises from the supposition that before a measurement
is made, particles exist in a sort of limbo state, being neither
here nor there while retaining the possibility of either.

Once an observer intervenes, the particle is forced to "choose" a
state ? to settle on one possibility, eliminating the other
options. Thus, the state of the particle is altered by the act of
measuring it.

Usually the small difference caused by this back-action is
dwarfed by the interference to particles caused by laboratory
imperfections. But for the first time, scientists have achieved a
quantum measurement with virtually no additional disturbance
beyond what quantum mechanics deems unavoidable.

The researchers, led by Jurgen Volz of the Université Pierre et
Marie Curie in Paris, reported their findings in the July 14
issue of the journal Nature.

In the new experiment, Volz and colleagues trapped a single atom
of rubidium in a cavity between two mirrors. They then shined
laser light on the trapped atom. What happened next depended on
which of two energy states the atom was in. In one state, the
atom would "ignore" the light, which would bounce back and forth
between the mirrors and eventually leak to a detector beyond the
mirrors.

In the second state, the atom would absorb and re-emit the light
photons in a process called scattering. Scattering changes the
energy of the atom, and the researchers wanted to prevent that
effect; the only disturbance they wanted was from the effect of
their observation.

So they set the mirrors at a precise distance where the presence
of an atom in the second state would prevent the light from
bouncing back and forth between the mirrors. Instead, all the
light would reflect off the first mirror, leaving the cavity
dark. The light would hit a detector in front of the first
mirror.

In either case, the state of the atom could be determined without
causing the scattering effect.

"Experiments done before used atoms in free space and shined a
laser beam on them," Maunz told LiveScience. "They could tell
which of the two states the atoms were in, but they scattered a
lot of photons. In this experiment they managed to determine the
state of the atom without scattering photons."

While the researchers were able to limit this disturbance, there
will always be a certain amount of back-action caused by any
measurement.

Ultimately, Maunz said, the experiment could help point the way
toward
quantum computers, which would use particles as bits to run
complex calculations quickly.

"At the end of computation you have to read out which state [the
particle] is in," Maunz said. "If you can read it out without
disturbing the system, that's an advantage there."

You can follow LiveScience.com senior writer Clara Moskowitz
on Twitter @ClaraMoskowitz.
Follow LiveScience for the latest in science news and discoveries
on Twitter@livescience and
onFacebook.